The present invention relates to multi-frequency laser sources useful for spectroscopy or the like and in particular to a simplified multi-frequency laser source using a nonlinear crystal mixer.
Light sources having frequencies related to absorption lines of chemical species are important elements in instruments such as infrared spectrometers used for chemical analysis. For measurements that require determination of ratios of particular isotopes or the like, a single light source that provides multiple, precisely located frequencies would be useful.
Lasers are known to provide intense narrowband light signals of precise frequency. Unfortunately, conventional lasers cannot be used directly in many important spectrographic applications because they produce light frequencies in the 1 to 1.5 μm range (near infrared) and many spectrographic applications require light in the middle infrared range of approximately 2 to 8 μm.
It is known to convert short wavelength laser light into longer wavelength light using mixers such as periodically poled crystals. One example of such crystals, periodically poled lithium niobate (PPLN), can be used to generate harmonically related frequencies as well as sum and difference frequencies from a mixed laser light from two different lasers simultaneously applied along the axis of the crystal. Periodic polling refers to adjustment of the ferroelectric domains of the crystal on the periodic basis along the crystal axes. Such crystals with different periodicities can be obtained commercially, for example, from Isowave of Dover, N.J., USA.
In order to convert high-frequency laser light into lower frequency light useful for common spectrographic applications, the two lasers (termed the pumping and signal lasers) must be precisely controlled in frequency so that their frequency difference, as will be used for down conversion, equals the desired output light frequency. The frequency of the pumping and signal lasers must also be controlled to be compatible with an operating point of the periodically poled crystal determined by its polling periodicity. The operating point may be adjusted slightly by controlling temperature of the crystal.
When two precisely tuned, mid-infrared or far-infrared light outputs are desired in a spectrographic application, the use of a PPLN crystal for down conversion normally requires two crystals and four lasers, a first pair of lasers (pumping and signal lasers) illuminating a first crystal to produce the first far-infrared light output, and a second pair of lasers (pumping and signal lasers) illuminating a second crystal to produce the second far-infrared light output. Each of the pumping and signal lasers must be designed to produce the desired frequencies. Custom lasers for particular frequencies can be quite expensive.
Recently it has been determined that the operating point of a single PPLN crystal is sufficiently accommodating to allow slight modulation of one of the lasers while still remaining within the operating point of the crystal. See generally: Measurement Of 13CH4/12CH4Ratios In Air Using Diode-Pumped 3.3 μm Difference-Frequency Generation In PPLN, S. Waltman et al, published in Vertical-Cavity Lasers, Technologies for a Global Information Infrastructure, WDM Components Technology, Advanced Semiconductor Lasers and Applications, Gallium Nitride Materials, Processing, and Devi, Aug. 11, 1997, pp 37-38, ISBN: 0-7803-3891-X, INSPEC Accession Number: 5752681.
The frequency bandwidth provided by this modulation has been determined to be sufficient to illuminate the Q(1) line of 12CH4 and the R(0) line of 13CH4 and thus to permit a direct ratio measurement of these isotopes with only two lasers by absorption at these frequencies.
The Q(1) and R(0) lines relate to different energy states of excitation of the methane molecule. Normally absorption measurements to determine the ratio of gases compare the same lines (e.g. Q(1) of both species). By using different lines Q(1) and R(0), the lines of the different species 12CH4 and 13CH4 fall within the modulation width of this technique.
This modulation technique substantially simplifies the construction of a spectrographic instrument for measuring isotope ratios by allowing one crystal and two lasers to perform the task of two crystals and four lasers. One drawback to this technique, however, is that the limited modulation range requires measurement of two different absorption lines Q(1) and R(0) to characterize the concentrations of different isotopes and the extraction of concentrations from two different absorption lines requires accurate knowledge of the temperature of the sample to within as little as 100 micro Kelvin. Often temperature measurements of a sample at this accuracy are difficult to obtain.